The present invention relates to a compound loudspeaker drive unit comprising a high frequency (H.F.) drive unit mounted concentrically within a low frequency (L.F.) drive unit.
For practical reasons, conventional loudspeakers typically comprise at least an L.F. drive unit and an H.F. drive unit. The two drive units are combined via an electrical or mechanical filter such that each unit is constrained to operate only over its optimum frequency range. In other words, the L.F. drive unit may be constrained to operate only over the low frequency range, and the H.F. drive unit may be constrained to operate only over the high frequency range. However, the low frequency range and high frequency range may overlap at a mid-frequency range such that both units output sound at the mid-frequency range.
Typically, these units are physically separated from each other. For example, they may be mounted adjacent to each other on the front of a loudspeaker cabinet such that the center of the L.F. drive unit is offset by some distance from the center of the H.F. drive unit. However, such separation causes the apparent sound sources or acoustic centers of the L.F. and H.F. drive units not to be equidistant from the listener for all possible positions where the listener may be located. As a result, the loudspeaker undesirably has a sound radiation characteristic that is non-uniform in all directions. For example, when both drive units are simultaneously outputting sound at the mid-frequency range, the distance from the L.F. drive unit to the listener may be different than the distance from the H.F. drive unit to the listener. Consequently, the sounds produced from the L.F. and H.F. drive units will not reach the listener at the same time, and the sound will be non-uniform. The undesirable sound radiation characteristic also results from the fact that the directivity of the L.F. and H.F. drive units are not matched because their sizes and shapes are different. In other words, due to the differences in sizes and shapes, in any particular direction, the sound emitted from the L.F. drive unit may be different than the sound emitted from the H.F. drive unit.
In order to try to overcome the problems above, numerous solutions have been proposed in which the H.F. drive unit is mounted concentrically within the L.F. drive unit. In the most successful of these proposed solutions, the L.F. drive unit is a cone shaped diaphragm, and the H.F. drive unit is mounted at the apex of the L.F. drive unit. Such an arrangement is shown in FIG. 1 which corresponds to a figure of U.S. Pat. No. 5,548,657, which is incorporated herein by reference. In such an arrangement, by virtue of the fact that the shape of the L.F. diaphragm drive unit 21 acts as a waveguide for the sound radiated from the H.F. drive unit 27, the L.F. diaphragm drive unit 21 imposes directivity control upon the radiation of sound from the H.F. drive unit 27. In this manner, substantially matched directivities are achieved throughout the mid-frequency range in which both units contribute significantly to the radiated sound. Additionally, this arrangement is intended to bring both the H.F. drive unit 27 and the L.F. drive unit 21 into time alignment such that the sounds emitted from the L.F. and H.F. drive units 21 and 27 reach the listener at the same time.
The arrangement disclosed in U.S. Pat. No. 5,548,657 and other similar arrangements have several disadvantages. For example, the L.F. diaphragm drive unit 21 is made of a substantially stiff cone. Also, the stiff cone is supported at its outer edge by a flexible rolling seal 22 to allow the axial movement of the L.F. diaphragm drive unit 21 required for sound radiation. As shown in FIG. 1, the necessary shape of this seal 22 interrupts the smooth surface of the L.F. diaphragm drive unit 21 extending from the H.F. drive unit 27 towards the outside of the loudspeaker. As a result, the sound emanating from H.F. drive unit 27 confronts such interruption, and irregularities in the frequency response of the H.F. drive unit 27 occur based upon the location of a listener.
In addition, as mentioned above, placing the H.F. drive unit 27 at the apex of the cone shaped L.F. diaphragm drive unit 21 is intended to bring both units into time alignment. However, the filters typically used to combine the two units add differential delays to the signals applied to those units, therefore disrupting the time alignment achieved by physically positioning the H.F. drive unit at the apex of the cone.